I'm not sure that auto-trading accounts for even a slither of market action at the moment - though i could be wrong.
Any automated system will need to be able to adapt to market changes. A system that worked this time last year in a nice trending upwards market will most certainly be failing in times such as those we are in now.
If anything, trading against a computer might be more profitable. If all the computers start buying at the first sign of a breakout- you can just jump on board and enjoy the ride.
The problem is, not all computers will trade the same, just as all humans will not trade the same. Everyone each has their own profit targets/goals/loss thresholds. So we are just back to the beginning again. So the market carries on.
I don't think we have to worry about automated trading taking over for quite some time yet.